1. Field of the Invention
This invention relates cooling fan made of synthetic resin, and more particularly to an improved cooling fan formed as a single unit with a flywheel for a small, general-purpose air-cooled engine.
2. Description of the Prior Art
A cooling fan for a general-purpose small air-cooled engine to be used in industry, farming and so forth is conventionally made of cast iron or an aluminum alloy casting formed with a flywheel as a single unit. Recently, a cooling fan molded from thermoplastic synthetic resin material adapted to be secured to a side of the flywheel has been employed instead because there has been a demand to increase the number of cooling fins which has complicated the configuration of the cooling fins and made molding by metal casting difficult.
Referring to FIG. 4, there is shown, by way of an example, a conventional cooling fan of this type made of synthetic resin, which has been proposed by this applicant in Japanese utility model application Sho 55-70252.
In FIG. 4, the components including a flywheel body `a`, a fan of synthetic resin `a.sub.2 ` and a starting belt pulley `d` are joined together as a single unit by a plurality of bolts `g`. The reference symbol `h` represents a spacer. The flywheel body `a` and the fan `a2` are fixedly secured to a crankshaft `b` of an engine by a nut `e`. The fan `a.sub.2 ` secured to the flywheel body `a` rotates synchronously with the revolution of an engine, thereby functioning as a centrifugal blower which draws in cooling air and cools the cylinders and cylinder heads.
Other single-unit-type flywheel cooling fans comprising a cooling fan formed as a single unit with a flywheel which are different from the cooling fan of FIG. 4 have been proposed in the Japanese patent laid-open publication Hei 9-250348 or the Japanese utility model laid-open publication Hei 6-58125. According to the invention of the publication Sho 9-250348, a cooling fan made of synthetic resin is fixedly secured to a flywheel together with a starting belt pulley by a clamping bolt secured to a crankshaft. In the device of the publication Hei 6-58125, a cooling fan made of synthetic resin is fixed to a cast metal flywheel by a plurality of bolts.
Conventional synthetic resin cooling fans, such as those disclosed in the above-mentioned Japanese utility model application Sho 55-70252 (FIG. 4), the patent laid-open publication Hei 9-250348, or the utility model laid-open publication Hei 6-58125, have the disadvantage that the fans tend to be deformed as a result of softening caused by the heat produced in an engine and the negative wind pressure produced during running of the engine. As a result, the cooling fan `a.sub.2 ` bends backwardly toward an inner side (i.e., the cooling fin side) so that the cooling fan may come into contact with a wind guide plate or a cooling air duct. In the past, several proposals have been made in order to resolve this disadvantage. The spacing between the cooling air duct and the fan `a.sub.2 ` (FIG. 4) has been kept wide in one proposal from which unfavorably degrades the blowing characteristics of the fan `a.sub.2 `. Another proposal is to increase the thickness of the backboard `a.sub.3 ` of the fan `a.sub.2 ` or to dispose a reinforcement rib on a back wall of the backboard `a.sub.3 ` to increase the strength of the fan `a.sub.2 `. Still another proposal is to extend the fins toward the center to increase the rigidity of the fan `a.sub.2 `. These conventional cooling fan constructions, however, unfavorably increase the weight of the cooling fans and complicate the configurations of the fans.
In conventional cooling fans made of synthetic resin, the back wall of the backboard `a.sub.3 ` is flat, resulting in the disadvantages that it is easy for the backboard to slip, and that the fan is liable to be deformed during transportation when many fans are randomly packed into a box.